1. Field of the Invention
The present invention relates to organic-light emitting devices having a hole transport layer and to materials suitable for use in the hole transport layer.
2. Related Technology
A typical organic light-emitting device (OLED) comprises a substrate, on which is supported an anode, a cathode and a light-emitting layer situated in between the anode and cathode and comprising at least one polymeric electroluminescent material. In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the light-emitting layer to form an exciton which then undergoes radioactive decay to emit light. Other layers may be present in the OLED, for example a layer of hole injection material, such as poly(ethylene dioxythiophene)/polystyrene sulphonate (PEDOT/PSS), may be provided between the anode and the light-emitting layer to assist injection of holes from the anode to the light-emitting layer. Further, a hole transport layer may be provided between the anode and the light-emitting layer to assist transport of holes to the light-emitting layer.
Polymers used to fabricate the device are preferably soluble in common organic solvents to facilitate their deposition. One example of soluble polymers are polyfluorenes, which have good film forming properties and which may be readily formed by Suzuki or Yamamoto polymerization. This enables a high degree of control over the regioregulatory of the resultant polymer.
WO 99/20675 is concerned with a process for preparing conjugated polymers. 9,9-disubstituted fluorene units are disclosed where the substituent is selected from C1-C20 hydrocarbyl or C1-C20 hydrocarbyl containing one or more S, N, O, P or Si atoms, C4-C16 hydrocarbyl carbonyloxy, or C4-C16 alkyl(trialkylsiloxy). It is further said that the two substituents may form with the 9-carbon on the fluorene ring a C5-C20 ring structure or a C4-C20 ring structure containing one or more heteroatoms of S, N or O. Only an n-octyl substituent is exemplified.
J. Am. Chem. Soc. 2001, 123, 946-953 is concerned with blue-emitting hompolymers. Polymer 20b has the formula:

EP 1088875 discloses electroluminescent devices having phenyl anthracene-based polymers. An adamantane spacer group is incorporated into the polymer in order to increase Tg. Polymer 67 has a repeat unit of the following formula:

where R2═R3=4-methoxyphenyl. The polymer is said to be a luminescent material.
WO 02/092723 discloses blue emissive polymers. It is said that incorporation of 2,7-linked 9,9 diarylfluorene repeat units into the electroluminescent polymer increase the glass transition temperature (Tg). In particular, repeat unit of formula:

is disclosed, where preferably, each Ar is independently selected from the group consisting of an optionally substituted residue of formula:

wherein n=1, 2 or 3 and R is a solubilizing group or hydrogen. Particularly preferred groups R are hydrogen and optionally substituted alkyl or alkoxy, most preferably R is hydrogen or butyl.
WO 02/092724 is concerned with electron transport and emissive polymers. Polymers according to one aspect of the invention of WO 02/092724 have a first repeat unit comprising a group having formula:

which is substituted or unsubstituted and where n is 1, 2 or 3 and X is hydrogen, or an alkyl, aryl, heteroaryl, alkoxy, arylalkyl, alkylaryl, cyano, halide, haloalkyl, haloaryl, haloheteroaryl or alkoxy group. Preferably, X is hydrogen or an alkyl group. These polymers are said to have higher Tg values than polymers previously used as electron transport materials. A Tg of 175° C. is mentioned. On page 9, it is said that it is preferred that X is selected from methyl, ethyl, n-butyl, s-butyl or tertiary butyl.
U.S. Pat. No. 6,653,438 is concerned with conjugated emissive polymers. A monomer for incorporating into a polymer is disclosed having the following formula:

In Example P4, this monomer is incorporated into a polymer with a triphenylamine repeat unit and a second, different fluorene repeat unit.
WO 2004/023573 discloses a method of forming an optical device. According to the method, a first layer is formed over a first electrode and a second layer is formed in contact with the first layer. The first layer is rendered at least partially insoluble by one or more of heat, vacuum and ambient drying treatments before the second layer is deposited. The first layer is formed by depositing a first semiconducting material and the second layer is formed by depositing a second semiconducting material. It is said that where one or both of the first and second semiconducting materials are polymers, it is preferred that the polymers are conjugated it is further said that such [conjugated] polymers preferably comprise 9-substituted or 9,9-disubstituted fluorene, 2,7-diyl repeat units, most preferably optionally substituted units of formula:

wherein R and R′ are independently selected from hydrogen, or optionally substituted alkyl, alkoxy, aryl, arylalkyl, heteroaryl and heteroarylalkyl.
WO 2006/070185 relates to amine monomers and polymers. It is said that particularly preferred amine polymers include optionally substituted 2,7-linked fluorenes, most preferably repeat units of formula:
wherein R1 and R2 are independently selected from hydrogen or optionally substituted alkyl, alkoxy, aryl, arylalkyl, heteroaryl and heteroarylalkyl. It is stated on page 18 that preferred hole transporting polymers are AB copolymers or a “first repeat unit” and a triarylamine repeat unit. The “first repeat unit” is defined as being selected from arylene repeat units.
There is always a need to provide new and, preferably, improved materials for the hole transport layer in a light-emitting device. In particular, it is desirable to find new and, preferably, improved materials that can be used as a common hole transport layer for blue, red and green electroluminescent materials in a full color display.
As stated in WO 2004/023573, when forming the light-emitting device it may be preferable to heat the hole transport layer prior to formation of the light-emitting layer. More preferably, the hole transport layer is heated at above the glass transition temperature of the hole transporting material. As such, it is further desirable to provide a hole transport material with a low glass transition temperature (Tg) so that it can be heated to above this Tg.